Hidden velocity ordering in dense suspensions of self-propelled disks

Recent investigations of the phase diagram of spherical, purely repulsive, active particles established the existence of a transition from a liquidlike to a solidlike phase analogous to the one observed in colloidal systems at thermal equilibrium. In particular, an intermediate hexatic phase is observed in two dimensions. At variance with previous studies, we highlight the dynamical anomalies of dense active phases employing suitable parameters accounting for the observed spatial velocity correlations. The resulting information is encoded into a phase diagram evidencing the nonequilibrium features of self-propelled systems at a high density. First, we unveil the growth—with density and activity—of ordered domains where the particles' velocities align in parallel or vortexlike domains, extending the preliminary observation found in the phase-coexistence regime. Second, when activity is strong, the spatial distribution of the kinetic energy becomes heterogeneous, with high-energy regions correlated with defects of the crystalline structure. This spatial heterogeneity is accompanied by temporal intermittency, with sudden peaks in the time series of kinetic energy. The observed dynamical anomalies cannot be detected by considering only the structural properties of the system and are exquisitely nonequilibrium peculiarities absent in dense equilibrium colloids.